The present invention relates to a fuel injection system for a two-stroke engine. In particular, the present invention concerns a fuel injection system for a two-stroke engine with crankcase scavenging, with at least one transfer duct. The transfer port to the transfer duct, which opens into the cylinder, is controlled by the engine piston. A fuel injector is positioned in the transfer duct, the injection jet from the injector being directed onto the side of the piston crown that is proximate to the combustion chamber, the axis of the injection jet subtending an angle of less than 90xc2x0 with the axis of the piston. The injection jet is directed, for the most part at least, onto that half of the piston crown that is opposite the exhaust port.
European Patent No. 302 045 B2 describes a two-stroke engine in which the injector is configured as a multi-orifice injector, and in which the injection process for the range of higher engine speeds begins before the transfer port of the transfer duct is uncovered by the piston, so that some of the fuel can be pre-vaporized in the transfer duct. This is necessary, in particular, if the amount of time required for injection exceeds the amount of time for which the transfer ducts are open, as may be the case at very high engine speeds. In such a case, however, one disadvantage is that essentially injection takes place radially to the cylinder wall, and in the direction of the scavenging gas flowing into the cylinder by way of the transfer ducts. When this happens, it is scarcely possible to avoid unburned fuel flowing out of the combustion chamber into the exhaust, and this in its turn results in a loss of fuel and increased hydrocarbon emissions.
In order to prevent or reduce the fuel-air mixture from flowing through the combustion chamber into the exhaust to the maximum possible extent, in a modified version of the two-stroke engine described heretofore, European Patent No. 302 045 B2 proposes that the piston be configured as a deflector-crown piston that has a rounded, concave deflector surface on the same side as the transfer port. The jet from the injector is directed, at least in part, onto said deflector surface. (See also Austrian Patent No. 3 394 U1).
U.S. Pat. No. 4,779,581 describes another two-stroke engine, in which fuel is injected in the same direction as the scavenging gas that is flowing into the cylinder. In this engine, the fuel is injected in the direction of the spark plug, away from the top surface of the piston.
It is also known that low-pressure injectors can be used. Low pressure injectors introduce the fuel directly into the combustion chamber when the piston has uncovered the exhaust duct or transfer ducts. (See, e.g., U.S. Pat. No. 5,762,040, German Patent No. 39 13 629 C2, and German Patent No. 37 44 609 A1.) However, according to these publications, because the fuel is injected not into the transfer duct but rather directly into the cylinder through a separate port, injection can only take place once the piston has uncovered the injection port. Otherwise, the fuel mixture would be injected directly onto the piston skirt, which, on the one hand, would result in inadequate preparation of the mixture and, on the other, would result in the film of lubricating oil being washed off the piston and this, in turn, would increase the danger of damage being done to the engine.
The same difficulty arises with European Patent No. 302 045 B2 and Austrian Patent No. 3 394 U1. In both engine designs, the injector nozzles open out into the transfer ducts. Although the injectors are almost perpendicular to the piston skirt, namely, the side of the piston skirt that is most greatly stressed (the cyclic pressure and back-pressure side), in the case of advanced injection of fuel before the edge of the transfer port is uncovered by the piston, the film of lubricating oil is washed off the piston, thereby curtailing the service life of the engine.
In addition, German Patent No. 37 44 609 describes the use of at least two injectors, each injector having its own, dedicated fuel-supply. In this engine, it is possible to activate each injector separately as a function of the operating parameters of the engine.
According to U.S. Pat. No. 5,762,040, two direct-injection low-pressure injectors can be provided for each cylinder. These injectors inject fuel directly into the cylinder and are connected to a common fuel-supply system. As before, the injectors are directed essentially towards the exhaust duct, so that a not insignificant loss of fuel, and the concomitant escape of unburned gasoline, have to be taken into account.
As indicated, in each of the prior art two-stroke engine designs, the fuel injection systems do not provide reduced emissions across a wide range of engine operating speeds.
In addition, the prior art engines also experience a decrease in performance across a range of operating parameters in addition to experiencing an increase in unwanted exhaust emissions.
Therefore, it is an object of the present invention to provide a fuel injection system for a two-stroke engine that maintains or increases performance of the engine across a range of operating parameters while also reducing exhaust emissions from the engine by solving the problems of the prior art listed above.
Accordingly, one aspect of the present invention is the provision of at least two fuel injectors, disposed so as to be essentially parallel to each other and so as to subtend an angle of 20xc2x0 to 50xc2x0, preferably 35xc2x0, with the axis of the cylinder, the injector being directed towards the side of the piston crown that is proximate to the combustion chamber. The fuel injectors open out into the side transfer ducts, preferably one injector into one side transfer duct on the left-hand side and one on the right-hand side, adjacent to a rear boost port. The arrangement of the fuel injectors in the side transfer ducts means that the fuel that is added to the gas (air or a combination of air, vaporized fuel, and/or oil (among other components)) is always injected almost perpendicularly (i.e., across and partly against) to the gas flowing into the combustion chamber. This results in the greatest possible flow differential between the inflowing gas and the fuel that is injected, and results in superior, favourable mixing conditions. Arrangement of the fuel injectors in this manner also permits the complete (or nearly complete) vaporization of the fuel and also prevents unburned hydrocarbons from being exhausted from the cylinder, since no unvaporized fuel can be sprayed into the exhaust.
According to the present invention, it is preferred that the injectors be configured as multi-orifice injectors, since these generally provide for finer vaporization as compared to single-orifice nozzles, given identical injection parameters (i.e., pressure, flow, etc.).
During development of the present invention, the inventors realized that, primarily in the range of greater engine speeds and loads when the amount of time available for injecting the fuel is very small, the fuel can be injected before the transfer port to the side transfer duct is uncovered. This means that some of the fuel can be pre-vaporized in the transfer duct. This ensures that sufficient fuel can be introduced into the cylinder, even at high engine speeds.
To be able to match the quantity of fuel that is introduced into the cylinder to particular demands, such as load and engine speed, the injectors may be activated independently of each other. Thus, in the partial-load range, only one of the two injectors may be active and, in contrast to this, when the engine is under full load, both the injectors may supply fuel to the cylinder. With such an arrangement, it also may be possible to have the injectors work in alternation when the engine is operating under partial load, so as to avoid localized overheating of the engine. To this end, one injector is activated for one cycle, and the other injector is activated for the subsequent cycle.
Another way of matching the quantity of fuel injected to the load on the engine is to use two injection valves of different sizes, which is to say, valves with different flow characteristics. The smaller of the two is designed to deliver fuel mainly when the engine is idling or running under partial load. The other is activated only when the engine is operating under a specific load or at a specific speed, so that the demand for fuel can be satisfied, especially when the engine is operating under full load.